This invention relates to integrated circuit amplifiers, and more particularly to differential amplifiers for driving high-voltage outputs.
High-speed telephone lines such as Digital-Subscriber Lines (DSL) need high-performance drivers. These line drivers must be able to rapidly drive signals on a copper pair telephone line. The line may be several kilometers in length and have a significant load that is much larger than the loads experienced in other systems such as computers and local-area networks.
Design of such DSL line drivers is thus quite challenging. While most digital systems operate at 5 volts or less, DSL lines can be driven with 12-volt signals which is then stepped-up by a transformer to the 35-volt peak-to-peak required by telephone line. However, the more voltage is stepped up, the more current has to be driven, even though the power delivered is the same. This increased current makes lower voltage drivers more difficult. Additionally, when a large step-up is used, the noise requirements of the receive amplifier need to be very low, which is another difficulty.
Most semiconductor fabrication processes are targeted to manufacture of 5-volt, 3-volt, 1.8-volt and lower devices. Applying a 12-volt power supply to such low-voltage devices would cause unwanted breakdowns such as drain-to-source punch-through of complementary metal-oxide-semiconductor (CMOS) transistors. Physical damage to the integrated circuit could result. Thus special design techniques must be employed when designing high-voltage line drivers using low-voltage transistors.